What should I do if I forgot to include a copy of the Schedule K-1 Form 1041 I received when I mailed my AMENDED 2018 tax return (the K-1 was the reason for amending)?

Wait.  If the IRS writes to ask for it, respond promptly with the requested information and send it to the person handling your amendment at the address provided on the notice.  

Do not send another amended form or a letter, as it will just make matters worse.

Thank you so much; this was very helpful.

My pleasure to help!